It is often desired to monitor the rotational speed of rotary machines, such as fans. When fans are used to cool electrical equipment, for example computers, it is very important that the fans operate at or above some minimum acceptable speed. On occasion the fan may slow down, or even stop, which may cause the computer to overheat if the reduction in fan speed is not noticed early enough. Thus it would be quite helpful if the operator could be alerted if the fan speed drops below the minimum acceptable speed. This would warn the operator of an impending problem before the temperature within the computer could rise as a result of the fan speed problem.